In prior mechanical speed change gearing systems which were shiftable under load, synchronization elements were required to match the rotating centrifugal masses on the drive side (principally the engine) with the rotating masses on the driven side. Heretofore, such synchronization elements comprised multiple disc clutches for each speed in the speed change gearing. With an engine of great centrifugal mass and/or high rotational speeds, e.g. a turbine engine, the shifting elements were required to be of correspondingly large dimensions and high masses. This resulted in excessive transmission weight, high no load losses and inferior transmission efficiencies.
It had been suggested to employ a hydrodynamic clutch between the engine and the speed change gearing. Because the slip of a hydrodynamic clutch was a function of the torque load applied, hydrodynamic clutches could not be precisely controlled to effect proper synchronization of the rotating masses. Additionally, because the hydrodynamic clutch could not operate without slip, a mechanical bridging clutch was also required.